The present invention relates to a thermal head and a method of manufacturing the thermal head.
FIG. 9 shows a partial cross sectional view of an example of a thermal head in the prior art. In the thermal head in the prior art, an under-grazed layer 2 of glass or the like is formed on an electrically insulated ceramic base plate 1, such as alumina Al.sub.2 O.sub.3 ; a common electrode 4 and individual electrodes 3 made of a conductive material, such as gold (Au), are formed thereon; and further a heating body 5 formed of oxidized ruthenium (RuO.sub.2) is formed thereon.
Further, an insulating protective film 6 formed of a glass material, such as PbO--SiO.sub.2 --ZrO.sub.2, for example, is formed almost all over the surface. A printing media, for example, thermal-sensitive paper 8 is carried by a platen roller 9 while being pressed with respect to the insulating protective film 6 in order to be colored by thermal transmission of heat of the heating body 5 through the insulating protective film 6.
FIG. 10 shows a partial plan view of the thermal head in the prior art.
As shown in FIG. 10, the printing media, such as thermal-sensitive paper, is colored by applying prescribed voltage between the common electrode 4 and the individual electrode 3 to heat a dotted portion of the heating body 5 located between a common lead electrode 4a and the individual electrode 3, the common lead electrode 4a extending from the common electrode 4.
Therefore, the insulating protective film 6 operates as a mechanical and electric protective layer. For this purpose, the film requires a certain mechanical strength and electric insulation.
The thermal head in the prior art, however, has problems that the insulating protective film 6 is prominently abraded due to pigments included in a thermal-sensitive layer of thermal-sensitive paper by friction with the paper as the printing media, and that the mechanical strength and electrical insulation of the insulating protective film 6 is hampered.
Furthermore, in labeling paper, since labeling paper is thick, pressurization of a platen roller 9 tends to be set high in order to match well with the thermal head.
In this case, high pressurization of the platen roller 9 promotes the abrasion of the insulating protective film 6. On the other hand, it was found in an experiment that anti-abrasion due to the friction of the insulating protective film 6 depends greatly on the printing duty when the thermal head prints letters on a printing media, such as thermal-sensitive paper 8.
Namely, abrasion amount tends to increase when the printing duty is higher than the low rate. Affects suffered by the thermal head in case of higher printing duty than the low rate show the highest temperature distribution at the central portion when heated by the heating element. And when the printing duty becomes higher, heat generated especially by repetition of successive printing is apt to be stored by synergy of heating resistor therearound. As a result of the temperature reaching near a transition point of the insulating protective film 6, the insulating protective film 6 can not keep its proper hardness and becomes sensitive to mechanical stress, such as friction. Accordingly, the printing media, such as thermal-sensitive paper, carry the insulating protected film 6 while being pressed by the platen roller 9, and anti-abrasion of the insulating protective film 6 is jeopardized.
To solve this problem, a method of forming a solid film, such as Si--Al--O--N, was proposed in accordance with Japanese Laid Open Publication (KOKAI) No. 4-214367, for example. However, in case of the solid film, such as Si--Al--O--N, a technique of forming the thin film, such as spattering, is required. When a prescribed thickness of a film is desired, it takes much longer time to form the film and it is impossible to do so at a low cost because targeted material is a solid film of Si--Al--O--N. Also, when the solid film of Si--Al--O--N or the like is formed on the protective film by a printing technology, a problem of peeling off of a layer occurs by stress between the protective film and solid film.
Further, since the printing media, such as thermal-sensitive paper, are carried while being pressed to the insulating protective film 6 by the platen roller, the insulating protect film 6 is destroyed by static electricity due to friction electricity with the printing media. As a result, resistance value of the heating body becomes irregular, so that the printing becomes inferior.
Further, the insulating protective film 6 is corroded by the affects of sodium ion Na.sup.+ and potassium ion K.sup.+ included in thermal-sensitive paper, which causes a problem of electric corrosion to deteriorate the electric insulation.
Furthermore, in the thermal head in the prior art, width to contribute to actual coloring is 150 .mu.m relative to the width of 220 .mu.m in a cross sectional direction, for example, because the heating body 5 is formed with the printing technology. Namely, the thickness of the heating body tends to be thin from the center toward the cross section thereof, and accordingly, as the resistance value at skirt portions in the cross sectional direction of the heating body is higher relative to the center, consumption of the power is limited at a lower level.
The object of the present invention is to provide a thermal head and a method of manufacturing the thermal head to improve efficiency of coloring while keeping mechanical and electric durability of the insulating protective film in order to solve the problems mentioned above.